Add menu icon to icon font

Closes #186

- Adds the menu icon from Ghost's icon set
- Alters CSS to use that new icon

post.hbs

@@ -10,7 +10,7 @@
     <nav class="main-nav {{#if image}}overlay{{/if}} clearfix">
         {{#if @blog.logo}}<a class="blog-logo" href="{{@blog.url}}"><img src="{{@blog.logo}}" alt="Blog Logo" /></a>{{/if}}
         {{#if @blog.navigation}}
-            <a class="menu-button" href="#"><span class="burger">&#9776;</span><span class="word">Menu</span></a>
+            <a class="menu-button icon-menu" href="#"><span class="word">Menu</span></a>
         {{/if}}
     </nav>
 </header>